1. Field of the Invention
The present invention relates to a beam steering optical system. More particularly, the present invention relates to an optical system for steering or redirecting light beams in forward and reverse directions through an objective lens and a steering optical system. The present invention is also directed to an ophthalmic apparatus using such a beam steering optical system.
2. Description of the Related Art
In optical devices, such as scanning laser microscopes, an irradiating beam for irradiating the eye is directed along an irradiating path to x and y axis steering mirrors before being focused at a target within the eye. A detecting beam reflected from the target is imaged back along substantially the same irradiating path to the x and y axis steering mirrors before it is split off from the irradiating path and directed to a photodetector or an imaging device. More generally, an optical system used in a clinical setting may require one or more irradiating beams and one or more detecting beams to be all steered together using one or more mirrors. In such a system, the irradiating beam may be a diagnostic probing beam such as the red beam of a laser Doppler instrument, or a treatment beam, such as a point-focused surgical laser beam. Direct illumination of the eye may also be provided independently of the steering mirror or mirrors, for example, by flooding the eye with sufficient light to view the field of clinical interest in the eye. The detecting beam or beams reflected by the eye may be focused to produce a visual image, converted by a photodetector to a localized tracking image signal, converted to a tissue reflectance value used for laser intensity control, or processed in some other way. In such instruments, it is desirable for the treatment light beam and the detecting light beam to pass through steering mirrors to vary the field of treatment or observation, respectively.
One instrument of this type is described in U.S. Pat. No. 4,856,891. This patent discloses an eye fundus tracker/stabilizer using a common steering system to steer a narrow diagnostic or treatment light beam toward the eye and to receive light returning from the eye as a return image. The advantage of such a beam steering system is that by moving a steering mirror or mirrors to stabilize the position of the return image, the diagnostic or treatment light beam incident on the eye is automatically maintained at a stable location on the eye fundus bearing a fixed spatial relationship to the imaged area. However, when such a system is used to observe or treat a target on the fundus of an eye, the relatively intense diagnostic or treatment light input into the instrument to irradiate the eye scatters in the steering assembly, adding substantial noise to the extremely weak light returning from the eye. In addition, when it is desired to position a steering mirror and one or more stops confocal with the observed field or with the pupil of the eye, precision is required in locating or aligning the beam with respect to these elements. This structure complicates the problem of maintaining sufficiently distinct paths for the input light and the return light, and further compounds the noise and cross-talk problems.
U.S. Pat. Nos. 5,094,523 and 5,106,184 propose solutions to these problems. These patents disclose a two dimensional light steering apparatus comprising a pair of pivotable beam-directing elements, each having first and second faces. A beam traveling in a first direction is redirected by the first face of both elements, and a beam traveling in the second direction is redirected by the second face of both elements. The elements are preferably relatively thin planar mirrors (so called two-sided mirrors) which each steer the light about one of two axes. The first and second faces each perform a virtually identical purely pivotal steering motion to provide a wide field scan which is not occluded by system pupils. Steering in two dimensions may be employed for irradiating an eye and detecting light reflected therefrom through a common objective lens assembly. This structure allows highly efficient and jitter-free imaging, while providing effective input/output beam separation for such difficult applications as simultaneously treating and imaging the fundus of the eye.
However, the apparatus disclosed in U.S. Pat. Nos. 5,094,523 and 5,106,184 is complicated because they are designed for two-dimensional beam steering and two-dimensional imaging. For example, the irradiating beam path and the detecting beam path cross each other in the beam steering optical system. This limits one's flexibility in aligning the optical elements comprising the steering optical system itself and other systems such as the irradiating optical system (which produces a suitable shape for the irradiating beam) and the light detecting optical system which contains stops and detectors. Such a complicated structure is not needed for one-dimensional beam steering, used, for example, in a laser Doppler instrument for measuring retinal vessel blood flow.
Thus, there is a need for a beam steering optical system and an ophthalmic instrument using beam steering that is simple in structure which generates little noise and cross-talk between an irradiating beam irradiating the eye and a detection beam reflected by the eye. There is also a need for a beam steering optical system and an ophthalmic instrument using beam steering that maintains distinct paths for the irradiating light and the detecting light. In addition, there is a need for a one-dimensional beam steering apparatus which generates low amounts of noise and little cross-talk between an irradiating beam irradiating the eye and a detection beam reflected by the eye.